As technology has advanced one of the benefits has been a reduction in the size of electronic devices. One area that has benefited from this phenomenon is the field of communications. Miniaturization of various types of electronics has enabled the creation of smaller, lighter, and cheaper transceivers. Cell phones and portable radios have become smaller and capable of communicating over greater distances or using reduced power levels. Another transceiver application where this trend has been observed in recent years is the Common Data Link (CDL) program. CDL is a family of data links that provide full duplex, point-to-point digital microwave communications for the Intelligence, Surveillance, and Reconnaissance (ISR) community. CDL provides for the command, control and communications between the ISR sensor platforms and ground station processors. Smaller and lighter CDL electronics have greatly expanded the number of applications and platforms that can accommodate CDL electronics. Size, weight, and power sensitive applications such as Unmanned Aerial Vehicles and portable CDL data links are examples of more recent CDL applications.
As transmission power has increased between transceivers, the amount of isolation required between the transmitter and receiver in a typical receiver has also increased. Isolation between transmitter and receiver is typically used in a full duplex transceiver to keep the transmitter from jamming the receiver. Isolation is usually provided in the front-end electronics portion of a transceiver, typically referred to as the Radio Frequency Electronics (RFE). The RFE normally includes a power amplifier, diplexer, and Low Noise Amplifier (LNA); and may also include switches and filters, providing the front-end amplification and isolation for a data link. The RFE is the component of high power communication systems that has most resisted the trend of miniaturization. The reason the RFE has not shrunk significantly is primarily due to the largest component of the traditional RFE, the waveguide diplexer, which due to critical internal cavity dimensions, cannot shrink.
A diplexer can be used to achieve the sufficient transmit to receive isolation that enables a full duplex link, such as a CDL link, to use a single antenna. Since isolation on the order of 120 dB between the transmitter and receiver is typically needed in combination with a low insertion loss, usually no more than 3 dB, a waveguide filter diplexer is often used over other filter implementations. While providing superior isolation with low insertion loss, the relative disadvantage of the waveguide filter diplexer is that it can be significantly larger and heavier than other components within the RFE.